Also for the good square record we can get:
Square record: (-x^2, 0, d, x, d-x, d+x) [where (d-x) = a^2, d+x = b^2, prod = c^2]
Goal record: (-X^2, 0, D, X, D-X, D+X) [conventionally D-X = a]
d^2 - x^2 = c^2 =d^2 - c^2 = x^2 => x^2 = (d-c)(d+c)
Moreover, when you look at these (d+c) (d-c) values, you see that
d+c = 2D
d-c = 2X
So you can say that 4DX = (d-c)(d+c) = x^2